To increase the efficiency of industrial type furnaces fired by radiant tube burners, it is known to employ heat exchangers or recuperators to transfer heat from combustion gas exhausted from the burner to combustion air flowing to the burner flame. While known recuperators in the industry have increased the efficiency of industrial furnaces, there is still a demand for improvement.
One type of recuperator known in the art is that referred to as the "Plug-In Recuperator". One Plug-In Recuperator is disclosed in Canadian Pat. No. 1,154,371, issued Sept. 27, 1983 to Collier. This recuperator is plugged in to the exhaust leg of a U-shaped radiant tube burner. The burner is fired by gas and oxygen in the other leg of the tube. Combustion air is fed to the burner through the recuperator. While this plug-in recuperator provides a one-pass heat transfer operation, i.e. the combustion air flows in counter-flow to the flow of exhaust gas, the heat recovered by this type of recuperator is not considered satisfactory. One criticism of the plug-in recuperator disclosed in said Canadian Pat. No. 1,154,371, is that the recuperator extends past the wall of the furnace and is believed to draw heat from the load in the furnace. This is believed to limit the efficiency of the industrial furnace.
Another type of recuperator is that shown in U.S. Pat. No. 4,090,558 issued to Akama on May 23, 1978. This patent discloses the use of a recuperator for a radiant type burner where the recuperator is located outside of the furnace. This recuperator employs a two pass heat exchange system (parallel flow and counter-flow) between the combustion air and the combustion gas. The combustion air and gas enter the heat exchanger tangentially and follow a helically wound path through the heat exchanger so as to increase the distance travelled by the gas and air. This enhances heat transfer.
Another typical recuperator referred to in the industry is a radiant tube recuperator. In this type of recuperator, the burner and the recuperator are formed in the same tube.
While the previously described recuperators are typical of those presently in use in industry, there is a demand for increased efficiency of industrial furnace operation to reduce the energy consumption of industrial furnaces.